The invention relates to a method for controlling the cooling time in an injection molding process of an injection molding machine having an injection molding mold which forms at least one cavity for an injection molded part to be produced, with a melt being introduced into the cavity through an ingate.
The injection molding process of a conventional injection molding machine is in principle subdivided into various time phases, to be precise:                closing of the mold,        injection of the melt,        holding pressure time,        cooling time for the injection molded part.        opening of the mold,        removing of the injection-molded part.        
While the opening, closing and removal processes take up about 35%, the injection process 5% and the holding pressure phase 10% of the time profile of a cycle, the cooling time always takes up 50%. Since, nowadays, there is a requirement for ever shorter cycle times, development attention is being paid to shortening this time.
There are various possible ways to calculate the cooling time, and it is virtually impossible to distinguish between the mathematical equations. By way of example, the following calculation is known:
      t    cool    =                    s        2                              π          2                *                  a          eff                      *    In    ⁢                  ⁢          (                        4          π                *                                            T              M                        -                          T              W                                                          T              E                        -                          T              W                                          )      
tcool=cooling time [s]
s=average wall thickness of the molding [mm]
aeff=thermal conductivity
TM=melt temperature of the plastic
TE=mold removal temperature of the plastic part
TW=mold wall temperature
These or similar formulae are generally used in order to calculate the cooling time for a molding, or to estimate it roughly before production. The injection molding process is then controlled on the basis of this rough estimate.
Recently, a cooling time control process has become available whose purpose is to determine the shortest possible cooling times for each cycle during production in order then to automatically terminate the cycle at this moment, and to open the mold in order to remove the molding. In this cooling time control process, the wall thickness of the molding and the thermal conductivity of the plastic are used as input variables, together with the melt temperature and the mold removal temperature, once these have been determined. The critical point in this case is the maximum value of the mold wall temperature, which is determined in each cycle. At the same time, the time at which the mold internal pressure drops back to atmospheric pressure (1 bar) is also determined. These values are compared with the theoretical calculation and are then output to the machine control system as an optimized value in the form of a digital signal for opening the mold. However, in practice, it has been found that this method has scarcely made it possible to shorten the time at all. Just the choice of the various temperature sensors results in greater scatter, and in a greater measurement error.
In EP-A-0 704 293, a description is given of a method for controlling the temperature of units of an injection molding machine and molds for processing plastics.
It is pointed out that only the mold cooling or temperature control can be kept under specific control as heat dissipation from the mold. In particular, there is the necessity to control the time duration of the throughflow of the temperature medium in the mold in such a way as to compensate for disturbances of any kind that act on the heat content of the mold. Mentioned as a location for determining the temperature is that the average mold temperature is continuously measured during the entire duration of the cycle at the location that is subjected equally to the thermal loads of both the injected melt and the temperature control for the respective temperature control circuit, this location being located approximately in the region of the geometric centre between the mold contour and the cooling channel or area and in the region of the center between the cooling water inlet and outlet, at a sufficiently great distance from the mold contour, or, in the case of the cylinder temperature, in the region of the geometric center between the inner wall of the cylinder and the temperature control channel.
U.S. Pat. No. 5,411,686 relates to a method for achieving not only a constant value of the mold temperature but also a constant value of the melt temperature, and also a constant value of the temperature during the ejection of the article produced. The corresponding temperature sensors are located centrally in the mold.
The object of the present invention is to develop a method of the abovementioned type which leads to the cycle times being shortened.